Consistently use broadcastUpdate instead of broadcastCacheUpdate

[jeffposnick]

R: @addyosmani @gauntface
CC: @mischnic

Fixes #1289

This is just normalizing the runtimeCaching option key to match the namespace that's exposed by workbox-sw.

I'll make a corresponding change to the docs. The docs have the correct value already.

[workbox-pr-bot]

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File	Before	After	Change	GZipped
packages/workbox-background-sync/build/workbox-background-sync.prod.js	3.27 KB	3.27 KB	0%	1.39 KB
packages/workbox-broadcast-cache-update/build/workbox-broadcast-cache-update.prod.js	1.07 KB	1.07 KB	0%	573 B
packages/workbox-build/build/_types.js	41 B	41 B	0%	61 B
packages/workbox-build/build/index.js	2.52 KB	2.52 KB	0%	1.06 KB
packages/workbox-cache-expiration/build/workbox-cache-expiration.prod.js	3.41 KB	3.41 KB	0%	1.26 KB
packages/workbox-cacheable-response/build/workbox-cacheable-response.prod.js	594 B	594 B	0%	350 B
packages/workbox-cli/build/app.js	5.09 KB	5.09 KB	0%	1.86 KB
packages/workbox-cli/build/bin.js	2.32 KB	2.32 KB	0%	1.03 KB
packages/workbox-core/build/workbox-core.prod.js	6.40 KB	6.40 KB	0%	2.54 KB
packages/workbox-google-analytics/build/workbox-google-analytics.prod.js	2.07 KB	2.07 KB	0%	1.02 KB
packages/workbox-precaching/build/workbox-precaching.prod.js	5.18 KB	5.18 KB	0%	2.02 KB
packages/workbox-range-requests/build/workbox-range-requests.prod.js	1.64 KB	1.64 KB	0%	807 B
packages/workbox-routing/build/workbox-routing.prod.js	2.77 KB	2.77 KB	0%	1.25 KB
packages/workbox-strategies/build/workbox-strategies.prod.js	3.24 KB	3.24 KB	0%	1.01 KB
packages/workbox-sw/build/workbox-sw.js	1.46 KB	1.46 KB	0%	793 B
packages/workbox-webpack-plugin/build/generate-sw.js	6.41 KB	6.41 KB	0%	2.17 KB
packages/workbox-webpack-plugin/build/index.js	742 B	742 B	0%	470 B
packages/workbox-webpack-plugin/build/inject-manifest.js	8.48 KB	8.48 KB	0%	2.77 KB

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We are using 154% of our max size budget.

Total Size: 22.52KB
Percentage of Size Used: 154%

Gzipped: 9.01KB

[coveralls]

Coverage remained the same at 89.15% when pulling 30418b2 on broadcast-update-plugin-fix into 50fab16 on v3.