Fixed subtitle and summary for Pricing Indices.

1. Disambiguation of subtitle from "Price Indexes" to "Pricing Indices". The new title clearly means indexes to be used by vendors in formulae to establish pricing. The old title could have referred either to this or to indices that are derived from price statistics (such as the Consumer Price Index)
2. The new summary is more generic regarding potential reasons for choosing an index, and also clarifies that an exchange rate is a particular type of pricing index (usually the WM-Reuters Spot Index).