fix: initialFetchPolicy instead initialPolicy

[yevmoroz]

Fixing documentation for ApolloClient.defaultOptions.watchQuery.nextFetchPolicy custom function.
Considerations:

• Context that is passed as second argument doesn't provide initialPolicy, but initialFetchPolicy:
• Type definition raises error when initialPolicy is used
• Search through code shows initialFetchPolicy is used in many places
• initialPolicy is obsolete and not used at all

[apollo-cla]

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[phryneas]

Good catch - thank you for the PR!