RCS attribution corrected

The RCS guide previously described Rich Communication Services as "a protocol developed by Google." That was inaccurate, and it is now corrected.

RCS is an industry protocol standardized by the GSMA (Global System for Mobile Communications Association), adopted by mobile carriers and device manufacturers worldwide to modernise texting. RCS Business Messaging (RBM) builds on it to enable rich, interactive conversations inside the default messaging app on Android devices.

guide-rcs.md now matches the wording already used in the RCS overview. There is no API or behavioural change — this is a documentation accuracy fix.