fix some math formation #226

Job	Run time
Intra-doc links	4m 4s
Build target wasm32-unknown-unknown	3m 57s
Bitrot check	1m 31s
Build target wasm32-wasi	4m 7s
Book tests	3m 50s
Rustfmt	15s
Test on ubuntu-latest	9m 46s
Test on windows-latest	12m 41s
Test on i686-unknown-linux-gnu	15m 28s
Test on macOS-latest	11m 18s
Test on i686-unknown-linux-gnu with beta features	15m 35s
Test on ubuntu-latest with beta features	9m 54s
Test on i686-unknown-linux-gnu with nightly features	17m 46s
Test on windows-latest with beta features	12m 33s
Test on macOS-latest with beta features	19m 8s
Test on ubuntu-latest with nightly features	10m 46s
Test on windows-latest with nightly features	14m 0s
Test on macOS-latest with nightly features	11m 24s
	2h 58m 3s

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