fbcodereview: fix wrong help string for sl template {reviewers}

Summary:
Run `sl help templates`, then look for `{reviewers}`.
Reviewers is a copypasta of `{phabdiff}`.

```
    phabdiff      String. Return the phabricator diff id for a given sl rev.
    phabsignalstatus
                  String. Return the diff Signal status for a given sl rev
    phabstatus    String. Return the diff approval status for a given sl rev
    phase         String. The commit phase name.
    remotenames   :remotenames: List of strings. List of remote names
                  associated with the commit.
    rev           Integer. The repository-local commit revision number.
    reviewers     String. Return the phabricator diff id for a given sl rev.
```

Reviewed By: zzl0

Differential Revision: D66995768

fbshipit-source-id: c7500f3357fdd136f9b5887c7d95b68dedd4717a

eden/scm/sapling/ext/fbcodereview.py

@@ -137,7 +137,7 @@ def singlepublicbase(repo, ctx, templ, **args):
 
 @templatekeyword("reviewers")
 def showreviewers(repo, ctx, templ, **args):
-    """String. Return the phabricator diff id for a given @prog@ rev."""
+    """String. Return a space-separated list of diff reviewers for a given @prog@ rev."""
     if ctx.node() is None:
         # working copy - use committemplate.reviewers, which can be found at
         # templ.t.cache.