Fix typo in spack external debug msg (#17982)

lib/spack/docs/packaging_guide.rst

@@ -4308,8 +4308,8 @@ follows:
                return
            # This implementation is lazy and only checks the first candidate
            exe_path = candidates[0]
-           exe = spack.util.executable.Executable(exe_path)
-           output = exe('--version')
+           exe = Executable(exe_path)
+           output = exe('--version', output=str, error=str)
            version_str = ...  # parse output for version string
            return Spec.from_detection(
                'foo-package@{0}'.format(version_str)

lib/spack/spack/cmd/external.py

@@ -260,7 +260,7 @@ def _get_external_packages(packages_to_check, system_path_to_exe=None):
 
             if not specs:
                 tty.debug(
-                    'The following executables in {0} were decidedly not'
+                    'The following executables in {0} were decidedly not '
                     'part of the package {1}: {2}'
                     .format(prefix, pkg.name, ', '.join(exes_in_prefix))
                 )