codeowners : use slash prefix for root files [no ci] (#16210)

This commit adds a leading slash to the paths of root-level files
in the CODEOWNERS file.

The motivation for this is that these might otherwise match files
in subdirectories that have other/additional owners will override them.

Refs: #16209 (comment)

Author: danbev

CODEOWNERS

@@ -95,12 +95,12 @@
 /tools/tokenize/                        @ggerganov
 /tools/tts/                             @ggerganov
 /vendor/                                @ggerganov
-.clang-format                           @slaren
-.clang-tidy                             @slaren
-AUTHORS                                 @ggerganov
-CMakeLists.txt                          @ggerganov
-CONTRIBUTING.md                         @ggerganov
-LICENSE                                 @ggerganov
-README.md                               @ggerganov
-SECURITY.md                             @ggerganov
-requirements*.txt                       @CISC
+/.clang-format                          @slaren
+/.clang-tidy                            @slaren
+/AUTHORS                                @ggerganov
+/CMakeLists.txt                         @ggerganov
+/CONTRIBUTING.md                        @ggerganov
+/LICENSE                                @ggerganov
+/README.md                              @ggerganov
+/SECURITY.md                            @ggerganov
+/requirements*.txt                      @CISC