Small fix to DEVGUIDE.md

[dsyme]

No description provided.

[cartermp]

Flaky test?

2019-01-14T13:56:46.2418204Z Failed   TypeProvider.Disposal.SmokeTest2
2019-01-14T13:56:46.2419392Z Error Message:
2019-01-14T13:56:46.2419836Z    Check1, countDisposals() < i, iteration 1
2019-01-14T13:56:46.2420270Z   Expected: True
2019-01-14T13:56:46.2420622Z   But was:  False
2019-01-14T13:56:46.2421012Z 
2019-01-14T13:56:46.2421360Z Stack Trace:
2019-01-14T13:56:46.2421743Z    at Tests.LanguageService.Script.UsingMSBuild.TypeProviderDisposalSmokeTest(Boolean clearing) in D:\a\1\s\vsintegration\tests\UnitTests\LegacyLanguageService\Tests.LanguageService.Script.fs:line 1615
2019-01-14T13:56:46.2422206Z    at Tests.LanguageService.Script.UsingMSBuild.TypeProvider.Disposal.SmokeTest2() in D:\a\1\s\vsintegration\tests\UnitTests\LegacyLanguageService\Tests.LanguageService.Script.fs:line 1652
2019-01-14T13:56:46.2422663Z 

[dsyme]

Looks like it