Fix typos (#237)

internal/driver/driver_test.go

 	pprofCommands["weblist"].postProcess = nil
 
 	// Our mockObjTool.Open will always return success, causing
-	// driver.locateBinaries to "find" the binaries below in a non-existant
+	// driver.locateBinaries to "find" the binaries below in a non-existent
 	// directory. As a workaround, point the search path to the fake
 	// directory containing out fake binaries.
 	savePath := os.Getenv("PPROF_BINARY_PATH")

internal/graph/graph_test.go

 	}
 }
 
-// createTestCase1 creates a test case that initally looks like:
+// createTestCase1 creates a test case that initially looks like:
 //     0
 //     |(5)
 //     1
 	}
 }
 
-// createTestCase3 creates an initally empty graph and expects an empty graph
+// createTestCase3 creates an initially empty graph and expects an empty graph
 // after trimming.
 func createTestCase3() trimTreeTestcase {
 	graph := &Graph{make(Nodes, 0)}

internal/symbolz/symbolz.go

 
 // symbolizeMapping symbolizes locations belonging to a Mapping by querying
 // a symbolz handler. An offset is applied to all addresses to take care of
-// normalization occured for merged Mappings.
+// normalization occurred for merged Mappings.
 func symbolizeMapping(source string, offset int64, syms func(string, string) ([]byte, error), m *profile.Mapping, p *profile.Profile) error {
 	// Construct query of addresses to symbolize.
 	var a []string