Merge pull request #7140 from atugushev/fix-typo-in-docstring

Fix a typo in InstallCommand.__doc__

src/pip/_internal/commands/install.py

@@ -118,7 +118,7 @@ class InstallCommand(RequirementCommand):
     - Local project directories.
     - Local or remote source archives.
 
-    pip also supports installing from "requirements files," which provide
+    pip also supports installing from "requirements files", which provide
     an easy way to specify a whole environment to be installed.
     """