Fix heading label warning

Author: jmid

doc/qcheck-core/index.mld

@@ -5,7 +5,7 @@ The [qcheck-core] opam package contains two libraries:
 - The [qcheck-core] library for defining property-based tests
 - The [qcheck-core.runner] library for running property-based tests
 
-{1: The [qcheck-core] library}
+{1 The [qcheck-core] library}
 
 The [qcheck-core] library exposes two toplevel modules:
 
@@ -21,7 +21,7 @@ removing the need for having to hand-write shrinkers.
 file an issue if you encounter problems using either of the two
 modules.
 
-{1: The [qcheck-core.runner] library}
+{1 The [qcheck-core.runner] library}
 
 The entry point of the [qcheck-core.runner] library is the {!QCheck_base_runner} module.