Fix typo in command to log pip-diff failure

The pip-diff tool from vendor/pip-pop is used to determine stale
requirements. When pip-diff encounters an unexpected failure, a count is
logged using mcount from heroku/buildpack-stdlib.

Due to a typo, mount(8) was invoked instead of mcount, with an invalid
argument.

bin/steps/pip-uninstall

@@ -13,7 +13,7 @@ if [ ! "$SKIP_PIP_INSTALL" ]; then
 
 
     if ! pip-diff --stale requirements-declared.txt requirements.txt --exclude setuptools pip wheel > .heroku/python/requirements-stale.txt; then
-      mount "failure.bad-requirements"
+      mcount "failure.bad-requirements"
     fi
 
     rm -fr requirements-declared.txt