docs(spec): Fix MTLS scheme proto embedding (#1462)

Currently properties for `MutualTlsSecurityScheme` are not renderred
([source](https://a2a-protocol.org/latest/specification/#456-mutualtlssecurityscheme)):
<img width="1273" height="374" alt="image"
src="https://github.com/user-attachments/assets/be4c7dbb-749d-4391-aec8-fdd55c0dd625"
/>

This PR makes casing consistent for both mkdocs anchors and heading.

Tested via `mkdocs serve`:
<img width="1505" height="626" alt="image"
src="https://github.com/user-attachments/assets/348a6ef9-3868-43bf-8319-efe63092b95b"
/>

Re #1460 (comment)

Author: ishymko

docs/specification.md

@@ -980,7 +980,7 @@ For detailed security guidance on push notifications, see [Section 13.2 Push Not
 
 <a id="MutualTlsSecurityScheme"></a>
 
-#### 4.5.6. MutualTLSSecurityScheme
+#### 4.5.6. MutualTlsSecurityScheme
 
 {{ proto_to_table("specification/a2a.proto", "MutualTlsSecurityScheme") }}
 

specification/a2a.proto

@@ -613,13 +613,13 @@ message OpenIdConnectSecurityScheme {
 }
 // --8<-- [end:OpenIdConnectSecurityScheme]
 
-// --8<-- [start:MutualTLSSecurityScheme]
+// --8<-- [start:MutualTlsSecurityScheme]
 // Defines a security scheme using mTLS authentication.
 message MutualTlsSecurityScheme {
   // An optional description for the security scheme.
   string description = 1;
 }
-// --8<-- [end:MutualTLSSecurityScheme]
+// --8<-- [end:MutualTlsSecurityScheme]
 
 // --8<-- [start:OAuthFlows]
 // Defines the configuration for the supported OAuth 2.0 flows.