docs(rpc): clarify README wording and note formatting

[viktorking7]

• Add missing article in the description of requestedBlockNumber.
• Standardize note styling by changing Note: to **Note:**.

[cb-heimdall]

🟡 Heimdall Review Status

Requirement	Status	More Info
Reviews	🟡 0/1	Denominator calculation Show calculation 1 if user is bot 0 1 if user is external 0 2 if repo is sensitive 0 From .codeflow.yml 1 Additional review requirements Show calculation Max 0 0 From CODEOWNERS 0 Global minimum 0 Max 1 1 1 if commit is unverified 0 Sum 1

[refcell]

Closing since these changes aren't needed. Please re-open if you feel otherwise.