Fix incorrect metadata member name in Section 8.2.1

[kanywst]

Section 8.2.1 (Paginated Requests) says:

Furthermore, the PDP MUST declare support for the corresponding capability URN in its supported_capabilities metadata (Section 9.1.2).

There is no supported_capabilities metadata member. Section 9.1.2, which this sentence itself cross-references, defines the member as capabilities, and that is the name registered in the PDP Metadata Registry (Section 12.3). supported_capabilities appears nowhere else in the document, and it is also present in the published version.

Should be fixed as: capabilities.