Killian review: example docs issues

• cumulative_hydrograph_examples.rst - Should this text be removed from lines 22 – 24?:

plot:: :context: :include-source:

• percentile_calculation_examples.rst - Line 15: “separatly” should be spelled “separately”
• similarity_examples.rst - Line 7: “acros” should be spelled “across”
• meta/calculations.rst - line 8: "has" to "have" (See Suggestions to Authors 8th ed, page 53-54 [https://internal.usgs.gov/publishing/sta/documents/sta8.pdf])

All addressed in !87 (merged)