Fixed formatting errors in rmagic R() docstring

Things now mostly render correctly in both terminal and HTML versions of the
docs.

Note that there are still numerous warnings regarding these magics. At least
some of them appear to be from the debauched manner in which the docs are being
generated from the @magic_arguments. In addition to the warnings you get when
building the docs, if you look at the HTML source for, e.g., rmagic, you'll see
that the options are formatted in numerous ways. I get a "--" converted to an
en-dash (–) for example.

In case you're looking at this in a system that messes up the formatting of the
HTML code-point above, the en-dash is 8211.

IPython/extensions/rmagic.py

@@ -434,12 +434,12 b' class RMagics(Magics):'
         is printed if it would printed be when evaluating the same code 
         within a standard R REPL.
         
-        Nothing is returned to python by default in cell mode.
+        Nothing is returned to python by default in cell mode::
 
             In [10]: %%R
                ....: Y = c(2,4,3,9)
                ....: summary(lm(Y~X))
-       
+
             Call:
             lm(formula = Y ~ X)
 
@@ -456,9 +456,9 b' class RMagics(Magics):'
             Multiple R-squared: 0.6993,Adjusted R-squared: 0.549
             F-statistic: 4.651 on 1 and 2 DF,  p-value: 0.1638
 
-        In the notebook, plots are published as the output of the cell.
+        In the notebook, plots are published as the output of the cell::
 
-        %R plot(X, Y)
+            %R plot(X, Y)
 
         will create a scatter plot of X bs Y.
 
@@ -484,19 +484,19 b' class RMagics(Magics):'
         * If the cell is not None, the magic returns None.
 
         * If the cell evaluates as False, the resulting value is returned
-        unless the final line prints something to the console, in
-        which case None is returned.
+          unless the final line prints something to the console, in
+          which case None is returned.
 
         * If the final line results in a NULL value when evaluated
-        by rpy2, then None is returned.
+          by rpy2, then None is returned.
 
         * No attempt is made to convert the final value to a structured array.
-        Use the --dataframe flag or %Rget to push / return a structured array.
+          Use the --dataframe flag or %Rget to push / return a structured array.
 
         * If the -n flag is present, there is no return value.
 
         * A trailing ';' will also result in no return value as the last
-        value in the line is an empty string.
+          value in the line is an empty string.
 
         The --dataframe argument will attempt to return structured arrays. 
         This is useful for dataframes with