fix typo

Author: miyazakh

doc/dox_comments/header_files/signature.h

@@ -15,7 +15,7 @@
     WC_SIGNATURE_TYPE_RSA / WC_SIGNATURE_TYPE_RSA_W_ENC.
     The caller is responsible for ensuring the pointer refers to the correct
     type; this function cannot verify the actual runtime type of the object.
-    \param key_len If key is non-NULL, key_len Must be exactly sizeof(ecc_key)
+    \param key_len If key is non-NULL, key_len must be exactly sizeof(ecc_key)
     or sizeof(RsaKey) matching the sig_type. Passing any other value
     causes the function to return BAD_FUNC_ARG without dereferencing key.
     The conventional idiom is to pass sizeof(*key) at the call site.