crecord: fix typo in the help text

In the crecord help dialog, the toggle all option was wrongfully documented.
Instead of using 'a', one must use 'A' to toggle all the hunks. The crecord
header that is always displayed on the screen contains the right shortcut and
does not need to be changed.

mercurial/crecord.py

@@ -1363,7 +1363,7 b' can use crecord multiple times to split '
 the following are valid keystrokes:
 
                 [space] : (un-)select item ([~]/[x] = partly/fully applied)
-                      a : (un-)select all items
+                      A : (un-)select all items
     up/down-arrow [k/j] : go to previous/next unfolded item
         pgup/pgdn [K/J] : go to previous/next item of same type
  right/left-arrow [l/h] : go to child item / parent item