Fix grammar of check-shebang-scripts error

pre_commit_hooks/check_shebang_scripts_are_executable.py

diff --git a/pre_commit_hooks/check_shebang_scripts_are_executable.py b/pre_commit_hooks/check_shebang_scripts_are_executable.py
index aca0f77..937425b 100644
--- a/pre_commit_hooks/check_shebang_scripts_are_executable.py
+++ b/pre_commit_hooks/check_shebang_scripts_are_executable.py
@@ -36,7 +36,7 @@
         f'`chmod +x {shlex.quote(path)}`\n'
         f'  If on Windows, you may also need to: '
         f'`git add --chmod=+x {shlex.quote(path)}`\n'
-        f'  If it not supposed to be executable, double-check its shebang '
+        f'  If it is not supposed to be executable, double-check its shebang '
         f'is wanted.\n',
         file=sys.stderr,
     )