bpf: Fix string kfuncs names in doc comments

Documentation comments for bpf_strnlen and bpf_strcspn contained
incorrect function names.

Fixes: e913705 ("bpf: Add kfuncs for read-only string operations")
Reported-by: Stephen Rothwell <sfr@canb.auug.org.au>
Closes: https://lore.kernel.org/bpf/20250627174759.3a435f86@canb.auug.org.au/T/#u
Signed-off-by: Viktor Malik <vmalik@redhat.com>
Link: https://lore.kernel.org/r/20250627082001.237606-1-vmalik@redhat.com
Signed-off-by: Alexei Starovoitov <ast@kernel.org>

Author: viktormalik

kernel/bpf/helpers.c

@@ -3451,7 +3451,7 @@ __bpf_kfunc int bpf_strrchr(const char *s__ign, int c)
 }
 
 /**
- * bpf_strlen - Calculate the length of a length-limited string
+ * bpf_strnlen - Calculate the length of a length-limited string
  * @s__ign: The string
  * @count: The maximum number of characters to count
  *
@@ -3541,8 +3541,8 @@ __bpf_kfunc int bpf_strspn(const char *s__ign, const char *accept__ign)
 }
 
 /**
- * strcspn - Calculate the length of the initial substring of @s__ign which
- *           does not contain letters in @reject__ign
+ * bpf_strcspn - Calculate the length of the initial substring of @s__ign which
+ *               does not contain letters in @reject__ign
  * @s__ign: The string to be searched
  * @reject__ign: The string to search for
  *