rust: fix non-utf8 char in requirements.rs

Apparently Phabricator detect `rust/hg-core/src/requirements.rs` file as non
utf8 ‽, and mark it as binary.  During application it ended up being non-utf8
and this made Rust (and as a result heptapod) very angry::

    error: couldn't read hg-core/src/requirements.rs: stream did not contain valid UTF-8
      --> hg-core/src/lib.rs:11:9
       |
    11 | pub mod requirements;
       |         ^^^^^^^^^^^^

    error: aborting due to previous error

    error: could not compile `hg-core`.

The after "fixing", the file content has no character matching the following
regexp:

  [^0-9-a-zA-Z /(|).,{}!\[\]:"&=>?_*-;<`'#]

So we should be fine, unless Phabricator does something funny again.

Differential Revision: https://phab.mercurial-scm.org/D9444

rust/hg-core/src/requirements.rs

@@ -7,7 +7,7 pub enum RequirementsError {
     Io(io::Error),
     /// The `requires` file is corrupted
     Corrupted,
-    /// The repository requires a feature that we don�t support
+    /// The repository requires a feature that we don't support
     Unsupported {
         feature: String,
     },
@@ -55,7 +55,7 pub fn load(repo_root: &Path) -> Result<
 pub fn check(repo_root: &Path) -> Result<(), RequirementsError> {
     for feature in load(repo_root)? {
         if !SUPPORTED.contains(&&*feature) {
-            return Err(RequirementsError::Unsupported { feature })
+            return Err(RequirementsError::Unsupported { feature });
         }
     }
     Ok(())