Fix a lot of man7 links to be more specific.

Bug: https://github.com/android/ndk/issues/2113
Change-Id: Ifc3add91c39422985c63e895dc32443b8f736c9d
diff --git a/libc/include/unistd.h b/libc/include/unistd.h
index e623339..808568a 100644
--- a/libc/include/unistd.h
+++ b/libc/include/unistd.h
@@ -283,7 +283,7 @@
 int chdir(const char* _Nonnull __path);
 
 /**
- * [fchdir(2)](https://man7.org/linux/man-pages/man2/chdir.2.html) changes
+ * [fchdir(2)](https://man7.org/linux/man-pages/man2/fchdir.2.html) changes
  * the current working directory to the given fd.
  *
  * This function affects all threads in the process, so is generally a bad idea