That is valid code.
The backslash is a line continuation. Your code has two quoted runs of text; the runs appear like two strings, but are really just one string because Ruby concatenates whitespace-separated runs.
Example of three quoted runs of text that are really just one string:
"a" "b" "c" => "abc"
Example of three quoted runs of text that are really just one string, using
\ line continuations:
"a" \ "b" \ "c" => "abc"
Example of three strings, using
+ line continuations and also concatenations:
"a" + "b" + "c" => "abc"
Other line continuation details: “Ruby interprets semicolons and newline characters as the ending of a statement. However, if Ruby encounters operators, such as +, -, or backslash at the end of a line, they indicate the continuation of a statement.” – Ruby Quick Guide