llvm-doe: fix typo breaking "spack external find" (#23399)

Fixes the following error when 'flang' is found on the system:

   $ ./bin/spack external find
   ==> Error: name 'compiler' is not defined

var/spack/repos/builtin/packages/llvm-doe/package.py

@@ -251,7 +251,7 @@ def determine_variants(cls, exes, version_str):
                 compilers['c'] = exe
             elif 'flang' in exe:
                 variants.append('+flang')
-                compiler['fc'] = exe
+                compilers['fc'] = exe
                 compilers['f77'] = exe
             elif 'ld.lld' in exe:
                 lld_found = True